In the absence of any non–gravitational couplings, the expansion of the Universe is itself a source of particle production [1]},[2]}. For example, the equation of motion (EOM) for a
momentum mode \(\chi _k\)  of a free scalar
in the Friedmann Universe with the metric \(ds^2 = a^2(\eta ) \, (d\eta ^2 - d {\bf x}^2)\)  reads [3]}
\(\chi _k^{\prime \prime } + \omega _k^2 \chi _k =0 \;,\) 
